Angle sensors are used, for example, in the area of automobiles for determining a travel direction. The angle detection is usually based on a magnetic field measurement by a magnetic field sensor. The detected measurement values are subsequently evaluated. Based on the measured values indicating a detected magnetic field direction, an angle is calculated.
For magnetic field measurement, sensor elements such as AMR, GMR or Hall sensors are used which differ by two different pre-magnetization directions. If these elements are connected to a bridge, the result is a difference voltage dependence on the magnetic field direction following a sine function. If a second bridge is added whose sensor element preferred direction is orthogonal to that of the first bridge, the result is a second difference voltage offset by 90°, i.e. a cosine function. These two difference voltages form a vector describing a circular path for magnetic field rotation in the voltage plane. This circular path will be referred to as measurement circle below.
FIG. 13 shows a possible realization of a magnetic field sensor 1301 in the form of a GMR measurement bridge. The magnetic field sensor 1301 comprises first sensor elements 1302 aligned with a first preferred direction 1304, and second sensor elements 1303 aligned with a second pre-magnetization direction 1305. Four first sensor elements 1302 are connected to a bridge circuit. Also, four second sensor elements 1303 are connected to a second bridge circuit. The first measurement bridge is designed to detect a component of the first preferred direction 1304 of a magnetic field, and the second measurement bridge is designed to detect a second component of the second preferred direction 1304 of the magnetic field to be detected. The first measurement bridge is designed to generate a first bridge voltage Ux 1306 corresponding to the first component of the magnetic field, i.e. the component along the first pre-magnetization direction or preferred direction. The second measurement bridge is designed for generating a second bridge voltage Uy 1307 corresponding to a second component, i.e. the component of the magnetic field to be detected along the second pre-magnetization direction.
The principle of the rotational angle measurement is based on the fact that a two-dimensional coordinate system is sufficient for the determination of an angle. The measurement system provides an X value and a Y value related to an origin of the coordinate system, for example the voltages Ux, Uy of a measurement point shown in FIG. 13. From this XY value pair, the associated angle of the measurement point may be calculated by means of methods suitable for a microprocessor. If all measurement points are located on a circular path, the calculated angle describes the absolute position of the rotational angle exactly. If, for example, a magnet is rotated above two magnetic sensors, and if, for example, one sensor is orientated into the X axis and the second sensor into the Y axis, the sine and cosine components of the circular movement are detected. The arc tangent function atan y/x allows to conclude the angle. As the angle gives a direction of the measurement point with respect to the coordinate system, this application may be employed as an angle sensor.
FIG. 14 illustrates the principle of the angle measurement. An X component and a Y component are plotted in a right-angled coordinate system. A first component 1406, in this case the X component, is plotted in the direction of a first axis 1411a, in this case the X axis, corresponding to a detected magnetic field direction 1408. A second component 1407, in this case the Y component, is plotted in the direction along a second axis 1411b, in this case a Y axis. From the X and Y components detected, for example, by the magnetic field sensor shown in FIG. 13, an angle A of the magnetic field direction 1408 may be calculated. The direction vector of the magnetic field direction 1408 corresponds to a diagonal of a rectangle subtended through the X component 1406 and the Y component 1407. Thus, the angle A of the magnetic field direction 1408 may be calculated by an arc tangent calculation from the X component 1406 and the Y component 1407.
If, however, the measurement points are not located on a circular path, but on an inclined, offset elliptical path with non-orthogonal axes, there will be a deviation of the calculated angle from the actual angle of a direction to be detected.
Deviations from the orthogonality between the two bridge elements, differences in the bridge sensitivities and different offset errors result in a deviation from the ideal circular path. The general course of the path if elliptical, has an offset center point and an inclined axis position. The mentioned influences are basically dependent on age and temperature.
Manufacturing and assembly of the angle sensor also result in errors which must be eliminated in the application of the sensor element to guarantee a correspondingly high measurement accuracy of the angle. Three types of errors may occur.
An offset error causes an offset in the X and/or Y axis. Due to manufacturing and temperatures in operation, an offset must be expected. This leads to a displacement of the measurement circle.
An amplitude error causes an amplitude in the X and/or Y axis. Due to manufacturing and especially temperature, an amplitude error must be expected. This leads to a distortion of the circle into an ellipse, which, however, still has the main axes in the X and Y axes.
An angle error between the X and Y components will occur if the sensors are not positioned by 90° or if the sensors are not accurately constructed.
In summary, due to the sum of the occurring errors, the circle to be represented becomes a general ellipse which may be located offset in any angle around the origin.
FIG. 15 shows a distortion of the circular path to an elliptical path caused by influences. An errored X component 1506′ and an errored Y component 1507′ of a detected magnetic field direction 1508′ subtend a vector diagram from which an errored angle A′ of the detected magnetic field direction may be calculated. Due to the errored X component 1506′ and the errored Y component 1507′, the direction vector 1508′ does not describe a circle around the origin of the X axis 1411a and the Y axis 1411b, but an ellipse 1510′ around a center point of an errored X axis 1511a′ and an errored Y axis 1511b′. An origin 1512 of the circle coordinate system differs from an origin 1512′ of the ellipse coordinate system. In addition, the axes of the ellipse coordinate system 1511a′, 1511b′ are rotated with respect to the circle axes 1411a, 1411b. The errored ellipse axes 1511a′, 1511b′ can further comprise an angle deviating from 90° with respect to each other.
In order to reduce the offset error and the amplitude error and/or gain error, the solution is a fixed calibration of the offset and gain after manufacturing. However, this has the disadvantage that offset and gain errors occurring during operation cannot be compensated. Angle errors are currently not calibrated.
DE 10154153 A1 describes a solution in which only an offset compensation with an axis intersection method and an N points method are used. However, this requires a control loop with all related problems like settling, stability, etc.
DE 10154154 A1 uses an amplitude value of a resultant in order to manage a temperature offset compensation therefrom via a table. Again, neither gain nor angle error are corrected.
DE 10052609 A1 uses a third order polynomial for the offset compensation. This polynomial must be determined during manufacturing and is constant for the rest of the life span. Again, no gain or angle correction is performed.
Due to the lack of an automatic calibration possibility, each sensor cell must be put into operation and calibrated during manufacturing. In an expensive calibration, measurement points must be incorporated which, in the worst case, also are to be detected under various environmental conditions, such as various temperatures. This requires a test setup with a rotating magnetic field, as various angular positions are required. A digital block realized in hardware (HW) performs the angle calculation. This may be, for example, iterative methods such as the CORDIC (coordinate rotation digital computer) algorithm, multiplying methods or table methods. The chip calculates field amplitude and field angle which are read out via a sensor control chip interface. An external program evaluates the read out measurement pairs and determines corresponding correction coefficients. These are then written to a non-volatile memory in the sensor chip. As the calibration is performed only once, runtime influences and sensor installation influences are not compensated. In order to be able to maintain the specification across the temperature range, a complex and area-intensive temperature compensation circuit must be used, and the duration of the calibration measurements is significantly increased and thus made more expensive by the long temperature changing periods.